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Ejector Pins: Pushing Your Parts Around
Ejector pins are the ‘bouncers’ of the injection molding world. They apply a force to
eject a part from the mold, and in some cases can leave marks. At Protomold, our goal is to design and position pins to minimize their effect on your parts, and while Protomold typically determines pin placement, customers get to sign off on pin locations before an order is finalized.
Pins are located in the B-side mold half, the side in which the part will stay when the mold opens. Once the mold is opened, the pins extend into the mold cavity, push the part out, and then retract, allowing the mold to close and be refilled.
Protomold uses round ejector pins, and their placement depends on a number of factors. Obviously the shape of the part is one(see Figure 1). Factors like draft and texture of sidewalls and depth of walls and ribs can increase the likelihood that areas of
the part will cling to the mold. Resin
choice can also affect pin placement or size. Some resins are ‘stickier’, requiring more force for release from the mold. Softer resins may also require the use of more or wider pins to spread force and prevent puncturing or marring of the cooled
plastic.
Figure 1 - An example of the illustration Protomold provides early in the process of designing the mold so that the location and size of both the gate(s) and ejector pins can be approved.
In Protomold’s process, the ends of ejector pins are flat and perpendicular to the direction in which the pin moves. To be effective, the pins need a flat ‘pad’ to push against, and the surface of the pad must be perpendicular to the direction of pin movement. If the part surface at that location is textured, the smooth surface of the pad will be apparent. And if the surface of the part is not parallel to the flat end of the ejector pin, the cosmetic impact will be even more obvious.
In a traditional steel production tool it may be possible to machine the end of the pin to match the contour of a part surface that is not perpendicular to the direction in which the pin moves, producing a contoured pin. Protomold’s process, however, does not support the production of contoured pins. If a pin needs to act on a part surface that is not parallel to the pin-end, there will have to be a pad provided that is in the same plane as the pin-end rather than that of the part surface. Because it is in a different plane than the part surface, the pad may be raised slightly above the part surface at one edge or recessed slightly below the part surface at one edge. Configuring a pad that is slightly recessed into the part surface is the default configuration for pins on contoured surfaces.
A post gate produces an extreme example of a raised ejector pad (see Figure 2). In cases in which an edge gate cannot be used, resin is injected through an extension of an ejector pin channel. When the part has cooled, the ejector pin pushes against the resulting post and, in the process, clips off the runner. The post is typically removed from the finished part in a secondary operation.
Figure 2 - A post gate allows resin to be injected through an ejector-pin hole. When the part is ejected, a small
‘post’ of plastic is left on the part where the ejector pin is located.
In most cases, ejector pads (or the vestiges left by their removal) are on the non-cosmetic sides of parts. In some cases,
however, this may not be possible. Take for example the case of a clip formed using a pass-through core (see Figure 3). In this
case, because the clip increases the surface area of that side of the part, the ‘clip-side’ part surface will adhere
more strongly to its mold half. This will make that mold half the B-side. The clip would normally be on the cosmetic side of the
part, but its presence requires that ejector pads also be on that side of the part.
Figure 3 - The bottom of the clip’s ‘hook’ and blue face of the clip’s shaft will be formed by a pass-through core (shown by yellow lines) of the A-side mold half, which protrudes through a hole in the base of the part. The rest of the clip is formed by the B-side mold half.
All of the above cases assume that there are surfaces against which pins can push to eject parts from the mold. There are, however, some designs in which there are no such surfaces. Take, for example, a grate, in which all that faces into the B-side mold half are the tops of ribs. If the rib edges do not provide enough surface area for the pins to push against, the designer would need to add some bosses to act as ejector pads.
In most cases, ejector pin placement is a relatively minor concern in the early phases of part design. Protomold will propose
pin placement when an order is placed and present a pin and gate layout for customer approval. At that time, Protomold will address any questions or concerns and make changes, if necessary, to meet customer requirements.
Read past Protomold Design Tips.
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